ABSTRACT This proposal builds on the scientific premise that even though right ventricular (RV) function and female sex are major determinants of survival in pulmonary arterial hypertension (PAH), no RV- or sex steroid-directed therapies exist. The goal of the proposal is to identify novel and therapeutically targetable mechanisms by which 17?-estradiol (E2) exerts protective effects on RV function in PAH. We provide evidence that E2 exerts specific RV-protective effects via its receptor ER?, and suggest a new mechanism by which ER? activates bone morphogenetic protein receptor 2 (Bmpr2) signaling to upregulate apelin, a potent effector of RV contractility, whose regulation in the RV is not yet known. In addition to these direct effects, we show that E2 indirectly improves RV function by decreasing collagen content in the proximal pulmonary artery (PA), resulting in enhanced PA compliance and RV-PA coupling, defined as better RV adaptation to increased afterload. Based on these findings, we put forward the novel hypothesis that E2 improves RV-PA coupling in PH by ER?- dependent up-regulation of cardiomyocyte apelin and by reduction of collagen content and cross-linking in the proximal PA. We propose the following aims: 1) To establish that E2 attenuates RV pro-apoptotic signaling and improves RV contractile function and contractile reserve via Bmpr2-dependent increases in apelin, 2) To identify the contribution of ER? to mediating RV-protection, and 3) To demonstrate that ER? improves proximal PA compliance by increasing collagenase-mediated collagen degradation and by decreasing lysyl oxidase-mediated collagen cross-linking. We generated a novel ER? knockout rat that will enable us to study the role of ER? in PA banding models (SA1&2) and in the sugen/hypoxia model (SA3), thus avoiding pitfalls of prior studies of sex hormone signaling that were limited by lack of RV failure and by E2's confounding pulmonary vascular effects. These studies will be complemented by studies of Bmpr2-deficient rats or apelin- deficient mice in SA1, and Col1a1R/R mice resistant to collagenase-mediated collagen degradation in SA3. We will complement our in vivo studies with experiments in isolated heart preparations, isolated cardiomyocytes and RV tissues from PAH patients. The proposed studies are significant, since they will 1) identify ER? as a critical modulator of RV function, 2) establish a novel and therapeutically targetable E2-ER?-Bmpr2-apelin axis in the RV, and 3) identify collagenase-mediated collagen degradation and inhibition of lysyl oxidase-mediated collagen cross-linking as functionally important mechanisms of ER? in the proximal PA. Our studies are innovative, since they, for the first time, will provide a molecular basis for E2's RV-protective effects in PAH. They provide technical innovation by use of a newly generated ER? knockout rat model and a new highly selective ER? agonist, allowing for mechanistic dissection of ER?'s role in RV failure. Upon completion of the proposed studies, we will have identified ER? as a novel mediator of adaptive signaling in the RV. This will allow for development of new RV-specific, non-hormonal treatments for both female and male PAH patients.